Error when calling `scale_linetype` (where no error was present previously)

[bockthom]

I stumbled upon an error in the scale_linetype function and its internal invocation of discrete_scale. When calling the function with aesthetics = "edge_linetype", I observe the following error (which I suspect to be related to palette being NULL in this case):

Error in `as_discrete_pal()`:
! Cannot convert `x` to a discrete palette.

Note, however, that the error does not occur if aesthetics is omitted or changed, e.g., to linetype. Furthermore, it does not appear when we manually call discrete_scale with aesthetics = "edge_linetype" as soon as we provide a palette. Here is minimal code to reproduce the bug:

library(igraph)
library(ggraph)
library(ggplot2)
library(scales)

network = igraph::make_graph(c("A", "B"))

# works
plot = ggraph::ggraph(network)
plot = plot + ggplot2::scale_linetype(aesthetics = "linetype")
print(plot)

# works
plot = ggraph::ggraph(network)
plot = plot + ggplot2::scale_linetype()
print(plot)

# works
plot = ggraph::ggraph(network)
plot = plot + ggplot2::discrete_scale(aesthetics = "edge_linetype", palette = scales::pal_linetype())
print(plot)

# breaks
plot = ggraph::ggraph(network)
plot = plot + ggplot2::scale_linetype(aesthetics = "edge_linetype")
print(plot)

This error has not been present prior to the release of version 4.0.0 of ggplot2

---

Cc: @hechtlC @maxloeffler

[teunbrand]

Thanks for the report! What this issue exposes is that the fallback system for the absence of palettes is not thorough enough to cover all cases of non-standard aesthetics (e.g. edge_linetype instead of linetype). I've seen this emerge in some reverse dependencies and submitted PRs that mitigate the issue, but apparently it is more systemic than I realised. I think the solution is to fortify the fallback mechanism at the scale level, rather than the theme level.